The present invention generally relates to the manufacture of composite substrates based on semiconductors and in particular, those based on silicon, such as “SOI” (Silicon On Insulator) type substrates and even bulk silicon substrates.
More particularly, the invention aims to produce a substrate formed from a thin layer of single crystal silicon of suitable quality, or other semiconductor, on a solid base of high resistivity, typically greater than 1 KΩ·cm (the thin layer may be of any resistivity), such a substrate being described as “semi-insulating”.
These substrates are used, for example, in microwave applications, where one wishes to integrate active and passive components (in particular inductances) on the same substrate.
Recourse to semi-insulating substrates to produce circuits operating at high and very high frequencies (typically of the order of several gigahertz for so-called “MMIC” (Microwave Integrated Circuit) circuits), limits high frequency losses due to Foucault currents in the mass of the support. This is why the support is conventionally made from gallium arsenide AsGa, known for its good intrinsic properties and high resistivity, typically of the order of 10 MΩ·cm.
Another justification for the use of highly resistive substrates for such circuits is associated with the manufacture of integrated passive components forming part of these circuits, whose quality factor must be as high as possible at the frequencies concerned. Now, for very high frequencies, the quality factor drops in the presence of conducting or semiconducting elements in the neighborhood of the component.
The concern above is to allow for this, consisting, in classical SOI substrates, in producing a very thin insulating layer between the useful part of the substrate and the remainder of it. In fact, it is a question in this context of eliminating as far as possible any conducting parts at distances of a few hundred μm or more and thereby obtaining high resistivity throughout the thickness of the support.
In terms of substrates with an insulating support, “SOQ” (Silicon On Quartz) substrates are equally well known, and even “SOS” (Silicon On Sapphire) (Al2O3), where the support has the required high resistivity. However, such substrates may be difficult to obtain with good quality, often being contaminated with metallic impurities.
A good quality thin layer of silicon may also be difficult to obtain, despite whatever well known technology is used to manufacture it (hetero-epitaxy, forming of layer with bonding principally by molecular bonding). Associated problems can arise from the use of such substrates, in particular because certain manufacturers of integrated circuits may refuse to introduce them into their production line, either for reasons of contamination, or because their transparency to light makes them incompatible with some optical sensors used in industrial processes, in particular for metrology purposes, or even because the maximum temperature to which they can be exposed without damage makes them incompatible with certain processes.
It will also be noted that SOQ or SOS substrates of good quality are not available in large diameters.
Another approach could be to produce high frequency integrated circuits in substrates possessing a silicon support layer with normal resistivity (either solid silicon or SOI substrates), then to transfer these circuits to insulating supports. Such techniques could firstly be cumbersome, implying a double turnaround of the circuit, and secondly unsuitable for an industrial process (high cost price, in particular because of the loss of the original supporting layer or the necessity to remove this, and low yield).
Yet another approach might consist in using solid FZ (Float Zone) silicon substrates. The main problem with these substrates is that they are not available today in diameters greater than 150 mm. Another problem is their low residual oxygen content, which firstly makes them mechanically fragile and secondly, by limiting the formation of oxygen precipitates, limits their ability to trap metallic impurities (“gettering”).
In addition, the production of high resistance substrates in solid CZ silicon with high resistivity could be considered using a variant of the CZ technique known under the name of MCZ (“Magnetic Field Applied CZ”, or “CZ in a magnetic field”), which, however, would involve sacrificing the crystallographic quality of these substrates, making them unsuitable for the production of components in them. In particular, the crystalline quality becomes mediocre, in particular due to the presence of an excess of oxygen precipitates, a mass of holes-known under the acronym of COPS (Crystal Originated Particles), which would prevent the production of high quality circuits with an acceptable yield.
It will also be noted that in solid substrates of FZ or CZ silicon, the useful layer and the substrate mass forming the support for the useful layer would come from the same slice or the same ingot of silicon, which presents a degree of constraint for the designers. In particular, while the presence of faults in the mass of the ingot does not in general pose insurmountable problems, the presence of such faults in the neighborhood of the surface of the useful part of a substrate cut from such an ingot makes it unsuitable for its purpose and the rate of rejection is high.
Finally, one could consider producing such high frequency circuits on SOI substrates whose support layer would be in FZ silicon, or even on a substrate whose useful layer would be formed by bonding onto such a FZ silicon support layer, in order to obtain the same decoupling between the useful layer and support layer. However, one would still have problems, in particular the limitation of the diameter to 150 mm today.
In addition, PCT application WO00/55397 describes a process for the manufacture of a substrate in silicon starting from an CZ silicon ingot with a particular interstitial oxygen content 01, then using a thermal precipitation treatment, reducing the interstitial oxygen content to obtain a substrate that maintains a high resistivity while presenting grains of oxygen precipitates (suitable for the purpose of trapping metallic impurities) and suitable mechanical strength.
This document also describes the production of a substrate of the SOI type by the bonding of a useful layer onto such a high resistivity substrate and in this case the thermal precipitation treatment is preferably performed after bonding and also serves to strengthen the bonding interface. This technique however has certain problems. In the first place, it may in some cases be required not to subject the useful layer to excessive temperatures (particularly with useful layers in gallium arsenide AsGa or in indium phosphide InP, or even useful layers in pre-treated silicon implying limitations relating to the subsequent exposure to high temperatures) and such materials may then be incompatible with this known process.
Secondly, the fact that heat treatment necessary for precipitation must also be designed in the perspective of reinforcement of the bonding interface results in an obvious lack of flexibility, knowing that it could be difficult to produce a satisfactory compromise between the adequate treatment for the work on the bonding interface and adequate treatment for precipitation and stabilization. It must therefore be noted in this regard, the behavior of oxygen in the field of electrical donor generation and precipitation and growth of precipitant grains varies especially as a function of conditions (duration and temperature) of the different phases of heat treatment.
Finally and most importantly, in the case of an SOI, the interstitial oxygen content Oi and its behavior in the vicinity of an insulting layer of oxygen separating the base of the useful layer are poorly controlled, especially as concerns the way in which the oxygen is capable of outwardly diffusing into the oxide layer. Consequently, one risks having, without these phenomena being capable of being controlled: either an Oi content that is locally too low to obtain satisfactory formation of precipitates; or, on the other hand, an Oi that is locally too high, leading to a significant lowering of resistivity and/or to a stability defect in this resistivity (which could, in particular, drop at the time of subsequent exposure to elevated temperatures, for example during realization of the components).
Accordingly, improvements in these methods and substrates are desired.